As the popularity of portable computing spreads, the demand for compact devices, such as smart phones, head mounted display (“HMDs”), tablets, laptops, etc., increases. To meet this demand, it is important to continue to shrink the form factor of internal components of these portable computing devices. One such internal device is a camera module.
Convention camera modules consist of a lens system and image sensor having a finite number of pixels. The resolution of the image captured by the camera module is determined by the pixel count of the particular image sensor. For example, a 5 mega-pixel image sensor with 1.75-um pixels has an active image area (pixel array) of about 4.6 mm×3.4 mm, while the whole image sensor die is approximately 5.75 mm×5.75 mm. This requires a lens system having a diagonal size of about 5.4 mm (often determined by the size of the last lens element in the lens system) so as to cover the whole image field on the image sensor. With the thickness of the lens barrel and camera holder, the horizontal dimension of the camera module extends to approximately 7.5 mm. Furthermore, to accommodate the finite resolution of the image sensor, the lens system often needs to include several lens elements stacked on top of each other to correct for optical aberrations to achieve a reasonable optical resolution across the whole image field. This lens stacking contributes significantly to the vertical height of the camera module.
As can be seen from the above discussion, the overall camera module size in all three dimensions is substantially determined in part by the size of the image sensor die. Therefore, one way to miniaturize a camera module is through reducing the size of the image sensor die. Conventionally, this size reduction has been achieved by shrinking the size of the individual pixels in the pixel array while maintaining or increasing the pixel count of the overall image sensor. However, there are a number of disadvantages to this miniaturization approach.
First, smaller pixel sizes impose significant challenges in the design and manufacturing of the camera lens system. As pixel sizes decrease, there must be a corresponding increase in the optical resolution of the lens system to maintain the image quality (e.g., sharpness). Second, smaller pixel sizes decrease the image sensor sensitivity and often sacrifice low-light performance for size and resolution. Third, there is a practical limit in the physical size to which a pixel can be shrunk. Pixels are already approaching this threshold, despite continued demand for increasingly smaller camera modules.